The present invention relates to methods and systems for removing undesirable metals from iron-containing waste materials produced in the steelmaking industries, such as Electric Arc Furnace (EAF) dusts or mill scale. In particular, this invention describes methods and systems for removing one or more toxic metals as for example: zinc, cadmium, lead, and others from such iron-containing wastes and recycling the iron contained for its utilization in steelmaking.
The steel industry produces each year large quantities of wastes which desirably could be recycled and utilized after proper treatment for economic value and for compliance with environmental regulations, for example: Electric Arc Furnace (EAF) dusts, mill scale, iron ore fines, etc., with the EAF dusts and mill scale being typically the principal sources. Particularly the EAF dusts pose a difficult problem to steel producers regarding their handling and disposal due to the leachable toxic elements they contain (resulting in being classified by the U.S. Environmental Protection Agency as a hazardous waste). Each year the EAF steelmaking plants produce about 650,000 metric tons of EAF dusts, as reported in 1996.
Because of environmental concerns and costs, there is a continuing interest in developing commercially-viable treatments that have the ability to recover metal values, render the dusts less hazardous and/or gain beneficial use by re-utilizing the metals. Zinc is the contained metal value usually considered for recovery, followed by iron. The amount of zinc available for recovery is expected to increase in the future as galvanizing becomes ever more common for rust-proofing automotive steels.
There are numerous proposals in the literature for removing the toxic metals: zinc, cadmium, and lead from iron wastes, especially with increased regulation in the last decade, having the aim of recycling and/or safely disposing of the wastes. Few of these appear to have been used even as marginally acceptable commercially (primarily because of cost), and none have proven to be universally practical, even when limited to only EAF dust. See the article entitled xe2x80x9cTreatment Option for Carbon Steel Electric Arc Furnace Dustxe2x80x9d, by Arthur E. Morris et al, (University of Missouri-Rolla and Rolla Research Center, U.S. Bureau of Mines). See also U.S. Pat. No. 5,667,553 for a representative collection of background literature and patents (beginning with an Arthur D. Little, Inc. report xe2x80x9cElectric Arc Furnace Dust-1993 Overview, A Summary of Dust Generation, Status of Regulations, Current and Emerging Treatment Processes, and Processing Costsxe2x80x9d). These prior processes can be generally classified as pertaining to three categories:
(a) Removal of the toxic metals by hydrometallurgical processes. These methods require carrying out the chemical reactions in aqueous environments and have the objection of being costly, adding to landfill bulk and/of producing large volumes of polluting effluents.
(b) Agglomerating the dusts in combination with coal, coke or other hydrocarbon and heating and thereby reducing the metallic oxides at a highly elevated temperatures. Here the undesirable metals are evaporated and separated from the reduced metallic iron and condensed for their utilization or disposal;
(c) Agglomerating the EAF dusts and reducing them with a gaseous reductant, such as hydrogen, carbon monoxide or mixtures thereof, at high temperature in fixed or fluidized bed reactors and separating the undesired metals by condensation thereof outside the reduction reactor. These latter methods are preferred for in site treatment of the wastes.
The most used methods for the processing of electric-arc-furnace fines and the recovery of undesirable metals like zinc, lead and cadmium, involve the operation of horizontal rotary kilns, flat rotary hearth furnaces and grate furnaces, but up to date there have been few, if any, proposals for utilizing efficient moving bed reduction reactors for this purpose.
The following patents were found by applicants relative to the present invention: U.S. Pat. No. 4,673,431 to Bricmont; U.S. Pat. No. 5,013,532 to Sresty; and U.S. Pat. No. 5,470,375 to Greenwalt. These and the other patents and articles cited herein are incorporated by reference.
Bricmont (U.S. Pat. No. 4,673,431) relates to an electric arc furnace dust recovery process wherein pellets are formed from a waste dust and are charged to an oxidizing chamber 14 in an oxidizing atmosphere. The chamber is heated to quite high temperatures (e.g. 2700xc2x0 F., i.e., 1482xc2x0 C.), sufficiently to vaporize lead oxide and oxides of cadmium, potassium and sodium, if present, but not to vaporize zinc oxide or iron oxide. The vapors are drawn from the chamber with flue gases and delivered cooled to a bag house where flue gases are separated from solidified particles of the vapors. The residual oxidized mass is cooled after removal from the chamber and fed to a reduction chamber for further processing. Bricmont thus first separates the solidified particles of most of the undesirable metals from a residual oxidized mass of iron and then the iron and zinc oxides are reduced to metallic iron and zinc for further separation by volatilization of the zinc. The high temperature separation of zinc in its oxide form would appear to be unnecessarily expensive in view of the present invention as discussed below.
Sresty (U.S. Pat. No. 5,013,532) describes a process for recovery of metals from EAF dust wherein the raw material is charged to a furnace in the form of pellets, briquettes, granules or lumps and is heated sufficiently to permit vaporization and removal of the undesired metals by a flowing gas stream. To this end an excess of hydrogen gas is introduced into the furnace to reduce the metal oxides and sweep the vapors out of the furnace. The hydrogen gas containing the metallic vapors is cooled down with water and the condensed metals are separated in a bag filter. The main characteristics of Sresty is that hydrogen is the sole reducing agent and that hydrogen is regenerated by reaction of the zinc metal swept from the flue dust with the water during a reoxidation step.
Greenwalt (U.S. Pat. No. 5,470,375) describes a process for treating EAF dusts and petroleum refinery residues (with toxic metals) in a reduction reactor/melter gasifier combination where the refinery residues are fed to the melter/gasifier and the dust is agglomerated into xc2xd inch particles with lime or Portland Cement, xe2x80x9callowed to agexe2x80x9d, and then fed to a reductive reactor. Reducing gas is generated in the melter gasifier, sweeps through the reduction reactor, and carries off vapors or aerosols of zinc, cadmium and lead for subsequent separation and recovery.
One of the objectives of the present invention is to recover safely yet economically undesirable metals like zinc, chromium, cadmium, and mixtures thereof contained in EAF dusts, mill scale, iron ore fines, metallic powder, or other particulate iron-containing materials, particularly waste and other by-products of the steel-making industry. Another object of the invention is to recover the iron content of such materials, producing a pre-reduced product or direct reduced iron. Another object of the invention is to reduce the impact of the steel-making industry and to reduce the EAF dust deposits accumulating in the world in an environmentally acceptable manner.
Other objects and advantages of the invention will be evident to those skilled in the art or will be described in this specification of the invention and appended drawings.
The objects of the present invention are generally achieved by providing processes and apparatus for recovering undesirable metals from iron-containing materials and for recycling the iron content of said materials, including as one preferred embodiment a method comprising: sintering said iron-containing material producing sinter masses; breaking as may be needed such oxidized mass(es) into manageable sinter lumps; separating sinter lumps of a size above about xc2xc inch from sinter fines and recycling the latter; preheating, if necessary, said sinter lumps in a non-reducing atmosphere to achieve a temperature higher than about 650xc2x0 C. and preferably at least 690xc2x0 C., and more preferably equal to or higher than about 800xc2x0 C. (but normally less than about 1200xc2x0 C., to avoid particle sticking); introducing said hot sinter lumps to a reduction reactor; introducing a reducing gas within a temperature range preferably of about 850xc2x0 C. to about 1200xc2x0 C. into said reduction reactor, contacting said sinter lumps with said reduction gas within said reduction reactor at a temperature effective under the prevailing conditions to reduce said hot sinter lumps to metallic iron and to melt and sufficiently vaporize said toxic metals (specifically at least one of Zn, Cd and Pb) to separate such toxic metals from such lumps by entrainment into said reducing gas; withdrawing said reducing gas from said reduction reactor as an off reducing gas along with vapors and/or entrained molten aerosols of said undesirable metals, said off reducing gas exiting said reactor at a temperature equal to or higher than about 650xc2x0 C. and preferably higher than 900xc2x0 C. in order to avoid premature condensation of the heavy metals within the reduction reactor, in the associated ducting, or on the feed materials; cooling said off reducing gas to a temperature low enough to condense and solidify said vapors of undesirable metals; and, separating said condensed undesirable metals (which can be in the form of oxides or free metals, the oxide resulting from sufficient exposure to water and the latter requiring special handling to avoid pyrotechnic exothermic reoxidation in air).
To achieve a process economically suitable for treatment in a moving bed reactor, it is required to have a material in lump or other particulate form with sufficient mechanical strength in order to withstand high temperature, pressure and abrasion, especially within the vertical moving bed reactor.
In order to separate said condensed undesirable metals, a metals separation unit is used, which could be a bag house unit. When installation of a bag house is not suitable, the undesirable metals can be recovered by other means, such as by washing said off gas stream with water, thereby producing a sludge containing oxides of the undesirable elements. This sludge can be further treated and the undesirable metals recovered by means known in the art, as for example a clarifier and a filter.
As mentioned above, in order to have a material with sufficient mechanical strength, it is required to have a strong sinter product. In the steel-making industry, there are two common forms of deposited EAF dust and mill scale produced as by products, in a dry form and a wet form. When the dust and scale are recovered by scrubbers and separated from water as sludge, the iron-containing material is recovered in form of lumps and have sufficient strength to be sintered with a suitable carbon containing material (the usual carbonaceous material used being coke). When the iron-containing material is dry and in form of fines, then in order to have a strong sinter product, it is necessary according to another more specific aspect of the present invention to agglomerate said fines in a special manner.
To obtain sinter product with sufficient strength and desirable porosity to meet the requirements for use in a moving bed reduction reactor, the inventive process combines the EAF dusts with coke, lime, mill scale, and a typical commercial binder material, with an appropriate water content sufficient to agglomerate as pellets, preferably with a diameter equal to or smaller than xe2x85x9c in (9.53 mm). The preferred size of the pellets is about 1 mm to about 10 mm or slightly more. Recycled sinter fines can also be added as a component of the agglomerates. Such recycled fines are those screened particles which are less than about xc2xc inch (6.35 mm) in diameter.
The composition of the material to be sintered can be very broad, since EAF dust and mill scale vary from a steel-making plant to another. A composition has been found to be effective where the iron containing ingredients were 60% mill scale and 40% EAF dust. Tests for increasingly larger percentages of EAF dust are in progress and likely will result in sinter produced from 100% EAF dust (with no mill scale). In order to have a strong sinter product, it has been found best to have a ratio of lime to silica within a range of 2.2 to 3.0, and preferably 2.5.
As an illustrative embodiment of the invention, but not restricted thereto, the following composition has been found in a sinter product which has the desired properties:
As an example of the formulation of the material to be sintered, see the following list of ingredients:
As an example of the strength achieved by the sinter product according to the present invention, in a tumbler test ISO, 80% of the material gave a result of 6.3 mm or higher (where a good result for a sinter product is considered to require 70% of the material being 6.3 mm higher). This strength test was in accordance with the International Organization for Standardization""s xe2x80x9cInternational Standard ISO 3271, Iron Ores-Determination of tumble strengthxe2x80x9d in its third edition 1995-11-01: Reference number: ISO 3271:1995(E).
The following additional example illustrates the degree of removal of lead, zinc, and cadmium achieved when practicing this invention. The sinter product fed to the reduction reactor in this further test was:
After this sinter was reduced, the laboratory results showed the following modified composition:
In subsequent additional tests, it has been determined that in the case where the iron-containing particles fed to the reduction reactor are in the form of sinter lumps, some removal of the toxic metals can occur during the sintering portion of the process. For example, a significant majority (even as much as 60-70%) of the cadmium present in the particles is commonly carried off in the flue gas from the sintering step and even as much as 25% of the zinc can similarly be driven off. The same can be true of other undesirable metals to a greater or lesser degree. Thus, the off gas from the sintering step can be treated by a separate separation unit for recovery of the undesired metals from the flue gas, such as by a quench cooler and a small sludge separator or baghouse (or even by combining the flue gas from the sinter plant with the off gas from the reduction reactor to thereby utilize the off gas metals separator for both and thus avoid the need for a separate separator for the flue gas, if that would not be too detrimental to the upgrading and recycling of the reducing off gas).
In the foregoing tables, no cadmium is listed. The amounts of cadmium in that experimental run, if any, were apparently too small to be measurable in the sinter (and the concentrations in the pre-sinter materials not having been indicated). From the following results of a more recent run, the likely reason for the absence of cadmium in the foregoing tables becomes somewhat clearer.
In the more recent run, 0.4% wt. of CdO was present in the materials fed to the sinter plant. 0.04% wt. of CdO (even less if figured as Cd alone) was present in metals recovered from the off gas of the reduction reactor in the baghouse (which latter percentage dose not even include the separated DRI), and no measurable amount was found in the DRI product from the reactor (i.e. the Cd content, if any, present in the DRI was probably considerably less than 0.03%, which latter is the smallest amount of any element measured for that run). From this, it can be seen that whatever of the Cd that was not recovered in the baghouse for the reducing off gas, was previously carried off in the sinter plant""s flue gas.
In this more recent run, the chromium present in the materials fed to the sinter plant and the amount in the sinter lumps from the plant was about the same amount, namely on the order of 0.17% wt. of Cr2O3 (i.e. about 0.12% wt. of Cr); thus very little, if any at all, of Cr would have been present in the sinter plant""s flue gas. 0.01% wt. of Cr2O3 (i.e. about 0.009% wt. of Cr) was present in metals recovered from the baghouse of the off gas from the reduction reactor, and no measurable amount was found in the DRI product from the reactor.
Under the Mexican environmental toxic waste standards (i.e. the Mexican Official Standard for the Characteristics, Proceedings of Identification, Classification and Lists of Dangerous Wastes, which current standard is called: NOM-052-ECOL-1993), the upper limits for leeching of undesirable metals from DRI-type materials are 5 ppm for Cr, Pb, and Zn and 1 ppm for Cd. In the DRI produced by applicants"" foregoing method, the value calculated according to the aforementioned standard were 0.015 ppm Cr, 0.13 ppm Pb, 0.025 ppm Zn, and even less for Cd.
In this specification and in the accompanying drawings, the preferred embodiment of the invention is shown and described. Various alternatives and modifications thereof have been suggested, but it is to be understood that these are not intended to be exhaustive, and that many changes and modifications can be made within the scope of the invention. The suggestions herein described are selected and included for illustrative purpose only, in order that others skilled in the art will more fully understood the invention and the principles thereof and will thus be enabled to modify it in a variety of forms, each as may be best suited to the conditions of a particular use.